FILE - In this Sept. 12, 2019, file photo Democratic presidential candidates former Vice President Joe Biden, left and Sen. Elizabeth Warren, D-Mass., talk during a Democratic presidential primary debate hosted by ABC at Texas Southern University in Houston. Warren, a Massachusetts senator and leading progressive, has become an unlikely confidant and adviser to Biden, the presumptive Democratic presidential nominee. They talk every 10 days or so, according to aides to both politicians who requested anonymity to describe their relationship. (AP Photo/David J. Phillip, File)

In this image from video, Sen. Bernie Sanders, I-Vt., speaks during the first night of the Democratic National Convention on Monday, Aug. 17, 2020. (Democratic National Convention via AP)

FILE - In this Feb. 25, 2020 file photo, Democratic presidential candidates, Sen. Bernie Sanders, I-Vt., left, and former Vice President Joe Biden, talk before a Democratic presidential primary debate in Charleston, S.C. Political task forces Biden formed with Sanders to solidify support among the Democratic Party's progressive wing recommended Wednesday, July 8, that the former vice president embrace proposals to combat climate change and institutional racism while expanding health care coverage and rebuilding a coronavirus-ravaged economy. (AP Photo/Matt Rourke, File)